1. Technical Field
The present invention pertains to the rigging of sailboats and, more particularly, to an improved method and apparatus for achieving improved sail shape.
2. Discussion of the Prior Art
Sailboats rely on the aerodynamic force produced by the longer path and consequently higher speed of air flow over the convex leeward surface of the sail relative to the path across the concave windward surface. The pressure caused by the faster leeward flow is less than that caused by the slower windward flow, and the pressure differential generates a resultant suction force acting perpendicular to the sail to produce up to sixty to eighty percent of the total propulsive thrust generated by the sail.
Since this concept was first realized in the nineteenth century, sail design has evolved from essentially flat plates capable simply of being blown downwind to the complex shaping of modern mainsails. These incorporate luff and foot curves cut in strips of fabric in excess of straight lines, and seamed together with the excess fabric drawn up and aft into the sail to create a curvature or draft with a generally aerodynamic shape. Optimizing such designs has been limited with respect to achieving an efficient cross-sectional airfoil shape over the vertical extent of the sail by the straight flat attachment of the foot or horizontal lower edge of the sail to the rigid straight boom.
Triangular mainsails are attached all along the mast and boom and a backstay line runs from the top of the mast to the rearward end of the boom. The sail blooms under wind loading and takes a shape that resembles an airfoil over its upper portion (if properly trimmed) but is forced to return to a straight shape along its foot since it is securely fastened all along the rigid straight boom. For a triangular sail the center of effort (centroid) lies above the lower one-third of the sail up from the foot, and a greater portion of the sail thrust-producing area is in the lower portion near the aerodynamically inefficient straight boom-foot attachment. In addition, the maximum lateral deflection or draft of a traditionally rigged sail under wind loading is at about midheight, with much of the wider foot portion deflected significantly less.
Some flex in the mast or rigging mitigates the severity of the transition from the bellowed interior to the foot. The return to a near linear contour, however, especially at the largest and most critical portion of the sail, still results in significant lost propulsive thrust. Perhaps equally as detrimental, the center of pressure is displaced higher up the sail, increasing the overturning moment arm acting between the wind pressure and the water resistance and compromising boat handling. If the foot of the sail were given a curved airfoil shape, then the maximum deflection, or sail draft, would be at the foot, resulting in a considerably larger surface area to collect the wind. In addition, the center of pressure would be lowered as well allowing a fuller, deeper, more powerful sail shape to be used in heavier wind without producing difficult to handle and hydrodynamically inefficient heeling in the hull.
The concept of the sail as an airfoil and even the concept of boom shaping to capitalize on airfoil thrust has been discussed in the literature. Exemplary of such approaches are U.S. Pat. No. 879,986 (Tatchell) and U.S. Pat. No. 3,310,017 (Dyer). These proposals, however, have offered no guidance in selecting and shaping a boom to achieve a specifically efficient airfoil contour. A merely flexible boom will assume a pseudo-aerodynamic curvature to achieve a sail shape minimizing net force; however, the thrust efficiency will be poor. An airfoil improperly matched to the sailboat environment is susceptible to premature air flow separation over the leeward surface with an attendant sharp decline in propulsive thrust. Soviet Union Patent No. 1,512,858 (Pivkin et al) illustrates a four section hinged boom but no characterization of section length or hinge angle is provided to indicate shaping in correspondence with any particular aerodynamic profile.
A primary problem with the prior art sail booms is that no method or criteria to select and continuously approximate a specific airfoil shape matched to the sailing environment has been defined. Another problem is the complex and obtrusive rigging proposed to control even the very suboptimized boom shaping devices of the prior art. These are inconvenient and sometimes dangerous for sailors forced to maneuver around them to control the boat. Another difficulty is that the prior art approaches to sail-foot profiling require costly replacement of existing booms without a backfit modification option available.